In the related art, various types of speed reducers have been proposed (see, e.g., JP Patent No. 5801692B (Reference 1) and CN Utility Model No 203686077 (Reference 2)). The speed reducer disclosed in Reference 1 includes a case having a case body and a case cover, which are formed of a resin material, an input shaft (a worm wheel or a metal member), which is rotationally driven by an electric motor and has an eccentric shaft portion that is eccentric to an axis of the input shaft, an externally toothed gear wheel (gear plate), which is rotatably supported on the eccentric shaft portion, and an output shaft, which is rotatably supported on the case to be coaxial with the input shaft and has an internally toothed gear wheel having a number of teeth. The number of teeth in the output shaft is larger than the number of teeth of the externally toothed gear wheel, and the output shaft is engaged with the externally toothed gear wheel. In addition, a guide plate is disposed between the input shaft and the externally toothed gear wheel to guide the externally toothed gear wheel in such a manner that the externally toothed gear wheel is revolvable but non-rotatable.
That is, the case is formed with a pair of first guide recesses disposed at an interval of 180° around the output shaft, and the guide plate is provided with a pair of first guide protrusions, which are slidably inserted into the pair of first guide recesses, respectively. That is, since the first guide protrusions are inserted into the first guide recesses, sliding of the guide plate is guided in a first radial direction that is centered on the output shaft.
In addition, the guide plate is formed with a pair of second guide recesses, which are disposed at an interval of 180° around the externally toothed gear wheel to have an angle of 90° with respect to the pair of first guide protrusions, respectively. In the externally toothed gear wheel, a pair of second guide protrusions are provided to protrude so as to be slidably inserted into the pair of second guide recesses, respectively. That is, since the second guide protrusions are inserted into the second guide recesses, the sliding of the externally toothed gear wheel is guided in a second radial direction that is centered on the externally toothed gear wheel. Of course, the first and second radial directions are orthogonal to each other.
Accordingly, when the eccentric shaft portion (the input shaft) rotates, the externally toothed gear wheel revolves while moving in the first and second radial directions through the guide plate. In addition, when the externally toothed gear wheel makes one turn (revolution), the engagement portion engaged with the internally toothed gear wheel makes one turn, so that the internally toothed gear wheel (the output shaft) rotates by the difference in the number of teeth between the externally toothed gear wheel and the internally toothed gear wheel. Thereby, the rotation of the input shaft is sufficiently reduced in speed and is transmitted to the output shaft.
In particular, in Reference 1, a pair of fitting protrusions, which are disposed at an interval of 180° respectively to conform to the pair of first guide recesses, protrude from the surface of the case cover opposite the case body, and fitting recesses, into which the pair of fitting protrusions are respectively fitted, are formed in the surface of the case body opposite the case cover. Each fitting protrusion is formed so as to surround the corresponding first guide recess (and the first guide protrusion). In addition, the case (the case body and the case cover) is formed with a device hole for mounting a collar through which a bolt for fastening with a member to be attached is inserted. In addition, it has been proposed to dispose the device hole in the direction in which the guide plate is to rotate when an external load is reversely input from the output shaft in the vicinity of the fitting region of the fitting protrusions and the fitting recesses. Thereby, the external load reversely input from the output shaft is transmitted from the first guide protrusions of the guide plate to the fitting region of the fitting protrusions and the fitting recesses through the first guide recesses, and is distributed to the case body and the case cover. In addition, the external load transmitted to the case body and the case cover is transmitted to the member to be attached through the collar and the bolt.
In this way, it is possible to suppress the excessive load, which is reversely input from the output shaft, from being applied to the case body and the case cover, which are formed of a resin material.
In Reference 1, because stress concentrates on a cantilever-supported root portion of the first guide protrusion when the first guide recess, into which the first guide protrusion is inserted, receives the external load reversely input from the output shaft, a strength is required for the guide plate itself.
Thus, a need exists for a speed reducer which is not susceptible to the drawback mentioned above.